Buildings, towers and similarly heavy structures commonly are built on and around a steel framework. A primary element of the steel framework is the joint connection of the beams to the column. An improved structural joint connection is disclosed in U.S. Pat. No. 5,660,017. However, advanced stress analysis techniques and study of building collapse mechanisms following seismic and blast events (i.e., terrorist bombings) has resulted in the present improvement invention.
Briefly, the current technology teaches a joint structure for joining one or more beams in a supporting relationship to a column, including a pair of gusset plates spaced apart and sandwiching between them a column and a connecting beam or beams, with the gusset plates extending outwardly from the column along the sides of the beam(s). Of course, as taught in U.S. Pat. No. 5,660,017, the gusset plates may extend in both directions from a column so that they extend across the column, and connect two beams together, in a supporting relationship to the interposed column.
Fillet welds, possibly of multiple passes, are preferably used both in attaching the gusset plates to the vertical flange edges of the column and in the longitudinal welds attaching the gusset plates to the beam(s) or, alternatively, to cover plates attached to the beam.
Some of the “conventional” joint connection inventions in the prior technologies were characterized by unreliable performance of the joint connections. When such prior connections were loaded by severe moments and loads such as those caused by earthquakes, they failed. The Northridge earthquake in California in 1994 demonstrated that such prior joint connections were inadequate for resisting or carrying, (transferring), moments and loads caused by strong earthquake. Therefore, such conventional joint connections were also potentially unsuitable in the event of explosion and subsequent progressive collapse load conditions, severe weather and other potentially disastrous events. Under severe load and moment conditions, occasioned by such a potentially disastrous event, the forces and loads of the event would possibly cause the conventional joint connection to fail or perform poorly and unpredictably. The failure mode generally included one or more of: fracture of the welds, fracture of the metal of the beam or of the column, or the beam pulled divots out of the flange, (i.e., face), of the column.
In prior joint technology structures, the beam-to-to column joint connections exhibit insufficient strength and robustness, insufficient resistance to moments, insufficient resistance to inelastic strain levels of moment and axial tension, and insufficient ductility; demonstrating little or no continued strength beyond the yield point of the joint connections. Further, prior joint connection structures used more material (i.e., typically steel and weld metal) than was desired or needed, and required more labor than the current inventive joint connection for fabrication.
Over the last several years, there has been considerable additional concern as to how to improve the beam-to-column, and beam-to-beam joint connections so they will withstand explosions, blasts and the like as well as other related extraordinary load phenomena. Of particular concern is the prevention of progressive collapse of a building if there are one or more column failures due to terrorist bomb blast, vehicular and/or debris impact, structural fire, or any other impact and/or heat-induced damaging condition.
Column failures due to explosions, severe impact and/or sustained fire, have led to progressive collapse of entire buildings. An example of such progressive collapse occurred in the bombing of the A. P. Murrah Federal Building in Oklahoma City in 1995 and in the aerial attack on the World Trade Center towers in 2001.
Following the 1994, Northridge, Calif. earthquake, in addition to the invention set forth in U.S. Pat. No. 5,660,017, a number of other alternatives to resist joint connection failure, were suggested or adopted for use in steel construction design for improved seismic performance. For example, the reduced beam section (RBS), or “dog bone” joint connection, in which the beam flanges are narrowed near the joint connection has been considered. This alternative design reduces the plastic moment capacity of the beam allowing inelastic hinge formation in the beam to occur at the reduced section of the beam. This inelastic hinge connection is thought to relieve some of the stress in the joint connection between the beam and the column. An example is seen in U.S. Pat. No. 5,595,040, for Beam-to-Column Connection, which illustrates such “dog bone” connections. But, because the plastic moment capacity of the beam is reduced due to the narrowing of the beam flanges the moment load which can be sustained by the beam is substantially reduced.
Another alternative is illustrated by U.S. Pat. No. 6,237,303, in which slots and holes are provided in the web of one or both of the column and the beam, in the vicinity of the joint connection, in order to provide improved stress and strain distribution in the vicinity of the joint connection. Other post-Northridge joint connections are also identified in FEMA 350-Recommended Seismic Design Criteria for New Steel Moment Frame Building, published by the Federal Emergency Management Agency in 2000. All such post-Northridge joint connections have reportedly demonstrated their ability to achieve the required inelastic rotational capacity to survive a severe earthquake.
None of these alternative joint connections, however, provide independent beam-to-beam structural continuity across a column; such continuity being capable of independently carrying gravity loads under a “double-span” condition resulting from a column being suddenly or violently removed by, for example, explosion, blast, impact or other means, regardless of the damaged condition of the column; while also providing advantages in material, weight, and labor savings. Indeed, there are no additional and discrete load paths across the column in the event of column failure or joint connection failure or both. Additionally none of these alternatives, except the gusset plates used as taught in U.S. Pat. No. 5,660,017, provide any significant torsion capacity or significant resistance to lateral bending to resist direct explosive air blast impingement and severe impact loads. Torsion demands for the joint are created because the top flange of the beams is typically rigidly attached to the floor system of a building laterally, thereby leaving the bottom flange of the beam free to twist when subjected to, for example, direct lateral blast impingement loads caused by a terrorist attack.